Fluvial organic carbon cycling regulated by sediment transit time and mineral protection

Rivers transfer terrestrial organic carbon (OC) from mountains to ocean basins, playing a key role in the global carbon cycle. During fluvial transit, OC may be oxidized and emitted to the atmosphere as CO2 or preserved and transported to downstream depositional sinks. The balance between oxidation and preservation determines the amount of particulate OC (POC) that can be buried long term, but the factors regulating this balance are poorly constrained. Here, we quantify the effects of fluvial transit on POC fluxes along an ~1,300 km lowland channel with no tributaries. We show that sediment transit time and mineral protection regulate the magnitude and rate of POC oxidation, respectively. Using a simple turnover model, we estimate that annual POC oxidation is a small percentage of the POC delivered to the river. Modelling shows that lateral erosion into POC-rich floodplains can increase POC fluxes to downstream basins, thereby offsetting POC oxidation. Consequently, rivers with high channel mobility can enhance CO2 drawdown while management practices that stabilize river channels may reduce the potential for CO2 drawdown.

Detecting the Delayed Signatures of Changing Sediment Supply in Salt-Marsh Landscapes: The Case of the Venice Lagoon (Italy)

Many salt-marsh systems worldwide are currently threatened by drowning and lateral erosion that are not counteracted by sufficient sediment supply. Here we analyze the response of a salt-marsh system to changes in sediment availability and show that, contrary to what would have been expected, marsh dynamics in the vertical plane can be insensitive to large sediment supply. We integrate sedimentological, geochronological, paleoecological, geophysical, and chemical analyses of salt-marsh sediments accumulated over the past six centuries in the Southern Venice Lagoon (Italy), and suggest that a time lag exists between enhanced river-fed clastic sediment input and its signature in the salt-marsh succession. This time lag is likely caused by the stocking of the sediment along the margins of pre-existing marshes, which started to significantly expand horizontally – rather than accrete vertically – when sediment input increased. When sediment input drastically decreased, wind waves re-suspended the river-fed deposits and distributed them over the marsh platform, eventually allowing for vertical accretion. Understanding the response of salt-marsh systems to changes in sediment supply has important implications for the management of tidal landscapes and the prediction of their evolution under the effects of natural and anthropogenic forcings. Our results highlight that the study of ultra-recent sedimentary successions needs to be carried out on the basis of a deep understanding of specific depositional dynamics.

MANIFESTATIONS OF UNFAVORABLE GEOGRAPHICAL PROCESSES WITHIN THE SMOTRICH CANYON OF KAMENETS-PODOLSKY

The research allowed the authors to identify various types of adverse, destructive and erosion processes in the canyon of the Smotrych River. Now the ecological situation within the Smotrytsya canyon is quite difficult. The main factor in the deterioration of the situation is anthropogenic and man-made load: quarrying, garbage accumulation, unauthorized agriculture on the slopes of the canyon. Within the Smotric canyon, natural water and wind erosion are manifested, which in most cases are combined and the processes of destruction as a result of their action are intensified. Studies of erosion processes in the canyon of the Smotrich River have revealed a great variety of their manifestations. Karst processes are a typical manifestation of water erosion in the study area. Among the surface forms of karst, microforms (crates of various types) predominate. Individual karst funnels are less common. We discovered a karst cave located near Bilanivska embankment. Its area is 6 m2. The walls are covered with cracks, dominated by horizontal, less vertical, but they are larger. Turfed, covered type of karst can be found on the section where the river Smotrich encircles the Old Town in the direction of the village. Poodles. Lateral erosion is due to slope processes and erosion of the riverbed. It causes the migration of the riverbed, watercourse, the formation of river meanders, widens the river valley. Within the Smotric canyon, this type of erosion occurs in places where the shoreline is formed of clay soil. Inflows and outflows of water masses during the season also change the structure of the soil. Wind deflation and weathering within the canyon are manifested in those areas where rock remains and leads to the formation of various forms, are evidence of long-term wind erosion. Over the centuries, these forms can change, collapse. Linear erosion includes slope processes under the influence of anthropogenic activity and temporary water flows, which form furrows, ravines, ravines. Bilanivska embankment is characterized by the development of a system of ravines that descend to the river floodplain as a result of temporary water flows, during heavy rainfall, heavy rains, melting snow. They are small, but descend to the river. There are ravines as a result of concentrated storm currents, which are formed during heavy rainfall Transport erosion occurs throughout the study area. The result of such erosion is narrow and wide paths, dirt and hard roads made of limestone and artificially covered and filled with rubble. Zoogenic (pasture) erosion is quite common in Smotrich canyon. Grazing of cattle (cows), horses or small animals (goats, sheep) is carried out everywhere. The lack of special pastures, the inability to keep animals in a tethered way, leads to the fact that animals graze on the slopes of the canyon, in the coastal zone, in parks, and goats graze even on steep terraces. During the last few years, the recreational service in the form of walks along the riverbed on the BRDM-2 vehicle has been causing a destructive impact on the ecosystem of the Smotrich River. Summarizing the results of our research, we should point out other negative impacts from tourism activities using military equipment: gradual destruction of the river bottom, acceleration of erosion processes on rocky and silty slopes, increasing turbidity, changing water circulation in the river, changing gorge, near Novoplan the bridge actually washes its supports, introduces fuel and lubricants into the water and with the wheels of the car feces of large animals, the noise scares away birds that nest in the coastal zone of the river. Key words: unfavorable geographical processes, ecological condition, Smotrich canyon, Smotrich river, erosion, pollution, sewage, landscape complexes, anthropogenization.

The Role of Frost Processes in the Retreat of River Banks

The rate of bank retreat was measured using erosion pins on the alluvial banks of the rivers in the Podhale region (the boundary zone between Central and Outer Carpathians) during the hydrological year 2013/2014. During the winter half-year (November–April), the bank retreat was mainly caused by processes related to the freezing and thawing of the ground (swelling, creep, downfall). During the summer half-year (May–October), fluvial processes and mass movements such as lateral erosion, washing out, and sliding predominated. The share of fluvial processes in the total annual amount of bank retreat (71 cm on average) was 4 times greater than that of the frost phenomena. Erosion on bank surfaces by frost phenomena during the cold half-year was greatest (up to 38 cm) on the upper parts of banks composed of fine-grained alluvium, while fluvial erosion during the summer half-year (exceeding 80 cm) mostly affected the lower parts of the banks, composed of gravel. The precise calculation of the relative role of frost phenomena in the annual balance of bank erosion was precluded at some stations by the loss of erosion pins in the summer flood.

The role of lateral erosion in the evolution of nondendritic drainage networks to dendricity and the persistence of dynamic networks

Dendritic, i.e., tree-like, river networks are ubiquitous features on Earth’s landscapes; however, how and why river networks organize themselves into this form are incompletely understood. A branching pattern has been argued to be an optimal state. Therefore, we should expect models of river evolution to drastically reorganize (suboptimal) purely nondendritic networks into (more optimal) dendritic networks. To date, current physically based models of river basin evolution are incapable of achieving this result without substantial allogenic forcing. Here, we present a model that does indeed accomplish massive drainage reorganization. The key feature in our model is basin-wide lateral incision of bedrock channels. The addition of this submodel allows for channels to laterally migrate, which generates river capture events and drainage migration. An important factor in the model that dictates the rate and frequency of drainage network reorganization is the ratio of two parameters, the lateral and vertical rock erodibility constants. In addition, our model is unique from others because its simulations approach a dynamic steady state. At a dynamic steady state, drainage networks persistently reorganize instead of approaching a stable configuration. Our model results suggest that lateral bedrock incision processes can drive major drainage reorganization and explain apparent long-lived transience in landscapes on Earth.

Mantle flow and orography: the effect of basal lithospheric shearing and lateral erosion gradients on continental rifting

<p><span>Mantle plume-lithosphere interactions modulated by surface processes across extensional tectonic settings give rise to outstanding topographies and sedimentary basins. However, the nature of these interactions and the mechanisms through which they control the evolution of continental rifts are still elusive. Basal lithospheric shearing due to plume-related mantle flow leads to extensional lithospheric rupturing and associated magmatism, rock exhumation, and topographic uplift away from the plume axis by a distance inversely proportional to the lithospheric elastic thickness. When moisturized air encounters a topographic barrier, it rises, decompresses, and saturates, leading to enhanced erosion on the windward side of the uplifted terrain. Orographic precipitation and asymmetric erosional unloading facilitate strain localization and lithospheric rupturing on the wetter and more eroded side of an extensional system. This simple model is validated against petro-thermo-mechanical numerical experiments where a rheologically stratified lithosphere above a mantle plume is subject to fluvial erosion proportional to stream power during extension. These findings are consistent with Eocene mantle upwelling and flood basalts in Ethiopia synchronous with distal initiation of lithospheric stretching in the Red Sea and Gulf of Aden as well as asymmetric topography and slip along extensional structures where orography sets an erosional gradient in the Main Ethiopian Rift (MER). I conclude that, although inherently related to the lithosphere rheology, the evolution of continental rifts is even more seriously conditioned by the mantle and surface dynamics than previously thoughts.</span></p>

A mechanistic model for bedrock undercut from saltating bedload particle impacts

<p>Bedrock walls can be undercut by saltating bedload particle impacts that are deflected by alluvial cover. Continued undercutting of the lower wall creates an imbalance on the wall and may cause the upper part to collapse and to widen the whole channel. Compared with vertical erosion rates, less is known about lateral erosion (undercutting) rates that are thought to dominate when river beds are alluviated. Here, we derive an analytical model for lateral erosion by saltating bedload particle impacts. The analytical model is a simplification of the Li et al. (2020) numerical model of the same process. The analytical model predicts a nonlinear dependence of lateral erosion rate on sediment supply, shear stress and grain size, revealing the same behaviour observed in the numerical model, but without tracking particle movements through time and space. The analytical model considers both uniformly distributed cover and patchy partial cover that is implemented with a fully alluviated patch along one bank and a bare bedrock along the other. The model predicts that lateral erosion rate peaks when the bed is ~70% covered for uniformly distributed alluvium and when the bed is fully covered for patchy alluvium. Vertical erosion dominates over lateral erosion for ~75% and >90% of sediment supply and transport conditions for uniformly distributed cover and patchy cover, respectively. We use the model to derive a phase diagram of channel responses (steepening, flattening, narrowing, widening) for various combinations of transport stage and relative sediment supply. Application of our model to Boulder Creek, CA captures the observed channel widening in response to increased sediment supply and steepening in response to larger grain size.</p>